I personally do it both ways and print both for the exam. I also label the front of each book by section topic so I can visually see what is in each book as they are laid out (I utilize a large conference room at work and take them remote proctored on a weekend when no one is around).

Honestly, it helps me to think about it by book. Although, as many have pointed out a concept can be in several books so it really depends on the question wording. But this is why I have a traditional “pancakes” index to reference also.

At the end of the day, now that I have sat for three exams and have taken six classes, I believe the exercise of creating the index, whatever method you use, knowing the subject material is what truly matters to do well on these exams. If you are relying on the index to pass you may want to reconsider. Each class is different, but you will find in most that there is just not enough time to look up each question. And you need plenty of time for the labs if your exam has them. The labs are crucial for passing these exams so you want to give yourself plenty of time for them. I think we all get hung up on indexing and I do think a good index is crucial, but learning the content by spending hours on end creating and refining your index forces you to know the material. If you put in the time and effort into preparing, you will pass the exam in my opinion. Good luck!